Investment



Jul 1, 1941.

Pmcmr TH RMAL EXPANSION R. NEIMAN INVESTMENT Filed Feb. 7, 1936 600 800TEMPE/8A TU/Z E INVENT OR.

liaanzr A/E/MA/V ATTORNEYS' Patented July l 1941 INVESTMENT RobertNeiman, Louisville, Kym, assignor, by. mesne assi factoring Company,ration oil Ulric tions, that is refractory compositions utilized in themaking of molds for the casting of metallic tents, to The ColumbusDental Manu- Colnnbus, Ulric, a corpo= Application February 7, 1936,Serial No. 62,801

objects which must conform as nearly as possible to a predeterminedsize, shape, and detail.

As an example of the use which would be made of this material referenceis made to a dental casting, or filling, which'is cast of a gold alloyor the like. As will be appreciated, this dental casting, or the like,must conform very accurately to the form and dimension of the cavity tobe filled.

In the reconstruction of a tooth or a group of teeth, the dentistprepares the cavity in a skillful manner, fills this cavity with warmwax, carves wax down to desired shape, removes same; and attaches a someThis wax is then completely encased in a cementitious mass of a mixtureof investment with water. Upon setting to a hard mass, the practitionerproceeds to remove the sprue and places the mold in an oven or furnace.As this mold is heated the wax melts and then volatilizes, and uponcontinued heating the wax residues of carbon are gradually decomposedand volatilized, the latter process requiring a temperature in theneighborhood of 1100 F. Between this temperature and 1300 F. is thegenerally desired mold temperature for good casting. Upon reaching thistemperature the mold is removed from the furnace and the alloy is castinto the cavity left by the disappearing \vax pattern.

In this casting process the exact size of the desired inlay or the likeis not produced, due to the casting shrinkage of the alloy insolidifying and coolingto ,the solid state. This shrinkage isapproximately-1.25% (linear) and obviously must be counteratted by asimilar degree of expansion of the mold, on heating, in order toreproduce a casting of a predetermined dimension. Due to variousconditions and modes of preparing the wax pattern the exact degree ofmold expansion varies within the limits of 1% to 1.65%. The additionalexpansion is achieved when the mold expands on setting in amountsvarying from .1 to .6%' depending on its composition. Best practicedemands investments with thermal expansions of slightly better than 1%to approximately 1.25%.

From the above dissertation and from a pracskilled practitioner asimple, and sure method of making artistic and very accurate castings.

The investment should:

First.-Have a thermal expansion (room temperature to approximate 1300F.) in the neighborhood of 1.25% for a mix of painting or pouringconsistency. V

Second.Attain nearly all of this expansion at 1150 F. with very littleadditional expansion to 1300 F.

Third.-.-Have practically no variation in thermal expansion when mixedwith an amount of water from 7% above to 7% below, the amount prescribedby the manufacturer, a leaway of 14% approximately.

Fourth.--Expand at a fairly even rate throughout the entire heatingrange, the expansion curve never rising very much above the practicalknowledge of the subject there may be formulated a list of propertiesand niceties which the profession has long desired in a commercialinvestment. An investment meeting these detically straight line curve ofthe usual metallic container or ring of equal expansion.

FiftI .-'-Incorporate readily with water to a smooth creamy consistencywhich can be easily and readily applied to every nook and me ',ce of thewax pattern and will not fall away from the wax due to its slipperynature.

Sixth.--Possess enough setting expansion to make an accuratereproduction (about .1%) and then an additional amount of expansion toaid in compensating for shrinkages of wax pattern, when using thedirect" method, of .1 to 25%.

Seventh.-Set rapidly to a hard cementitious mass which will withstandhigh temperatures and rapid heating without cracking or distortion.

Eighth-Retain this hardness for some time so that it may be used andhandled, even the next day, if necessary, without undue danger of easyfracture of delicate parts.

Ninth-Possess adequate strength when hotto safely resist the suddeninrush of molten alloy when cast under usual pressure; and preferably topossess enough strengtht after quenching in water, to permit easyremoval in toto rather than to disintegrate and leave a tenaciously'ad-EZeventh.-Not change materially on standing I orshipping due to thesettling out of unduly heavy constitutents such as powdered heavymetals, necessitating packing under pressure.

sirable and necessary properties will give the '55 Also possessingconstituents of a similar chemical ,Still lacking in the third,

and physical nature to give a homogeneous mass when mixed with water andalso to withstand storage under usual conditions without unduedisintegration and separation of ingredients.

The principal ingredients of, what may be 5 called simple investmentcompositions, as heretofore used, has been a mixture of plaster, knowncommercially as plaster of Paris, gypsum, stucco, hydrocal, alphagypsum, and chemically as calcium sulfate (CaSO4. H2O) and a refractoryfiller or aggregate. The plaster in its usual form, of the hemihydrate(CaSOL /ZHZO) or any other of the forms of calcium sulfate, hydrated ordehydrated, which set with water or watery solutions to a hard mass,generally acts as the binder.

The filler or aggregate consists of any refractory material which aidsthe mold in withstanding the high temperatures. It is desirable to use afiller which will also aid in giving a high thermal. ex-

pansion. The most desirable filler is silica in its three principalmodifications: Quartz, tridymite, and cristobalite. Quartz is thecheapest and mostuniversally used. Obviously any other filler may beused that will contribute the de-' sired properties, such as aluminumoxide, chromite, magnesium oxide, and other oxides as well as silicates,clays, and the like. To give the desired setting and hardeningproperties accelerators and retarders of a well-known and commercialnature are added. Small amounts of graphite, and sometimes a little(fraction of a percent) clay, or the like are added to aid in producinga smother mold and thereby smoother castings.

It is well known that mixtures of plaster and silica will not give thedesired thermal expansion nor satisfy many of the other specifiedrequirements. In the endeavor to meet the desired specifications variousingredients have been added to the plaster and silica mixtures withvarying 40 degrees of success. cristobalite, for. example, when added inlarge amounts (50 to 70%) gives a material of high thermal expansion'andmeets many of the above' specifications. This composition does'not,however, meet, or meets but partially, the third, fourth, fifth,seventh, ninth, and tenth requirements. In a composition in which sodium(potassium or lithium) chloride is added the eighth, ninth, tenth andeleventh requirements are lacking.' Metallic ingredients when added failto meet the second, third, tenth and eleventh requirements. Boric acidhas also been added to the basic composition but is required in goodamounts such as 3 to 5% in order to give adequate expansion,

flfth and tenth requirements.

It is, therefore, the object of this invention to provide theprofession, and anyone desiringean but the composition. is

heated under identical conditions, mixed with same proportion of water,and which compounds are respectively with and without the addedingredient of this invention.

Simple investment compositions, as above noted, are well known and havebeen used for many years. The'general composition of these materials isfrom '20 to 50% plaster and 80 to 50% silica. The increase in silicacontent produces an increase in expansion but a decrease in strength.The proportions may be varied even beyond these limits depending uponthe exact technic used in producing the castings.

In the drawing the reference numeral I 0 in dicates the thermalexpansion (and contraction) of a mixture of 44% plaster with 56% silicaand indicates a thermal expansion of .64% between room temperature (70F.) to 1300 F. This investment was made with a water to powder ratio(W/P) of .26, that is 26 grams of water to 100 grams of the powdermixture. An increase of the W/P causes amaterial decrease in the thermalexpansion of this type of mixture.

It has been found that by the addition to the above simple investmentcompositions of small amounts of the chlorides of the alkaline earthmetals, known in chemistry as the metals of the stant at .26, will causean expansion of this investment of 1.1%. Even small amountsof thesechlorides Will cause an appreciable increase in,

thermal expansion which continues to increase as the amount isincreased. The upper limit of the amount of chlorides is perhaps 5% andeven more depending upon the amount of plaster used and upon the desiredexpansion and strength. It is not desired to limit the composition tothe inclusion of any certain specific amount as this depends upon theother ingredients, but a very good product can be made by the inclusionof 1% as shown by the graph. An increase in expansion will result if thesilica content is increasedand vice versa. Variation in the W/P ratio,however, no longer affects the thermal expansion so markedly asheretofore and will enable the, practitioner who does not take advantageof the modern measuring devices to achieve a great measure of successnevertheless.

artistic and accurate castingmade, principally, 60

by the disappearing wax pattern method, with' investment compositionswhich will meet the above requirements to a greater degree than anythingnow known or available.

.Other objects and advantages of this invention should be readilyapparent by reference to the following specification considered inconjunction with the accompanying drawing forming a partthereof, and itis understood that any modification may be" made in the exact structural7 details, there shown and described within the scope of the appendedclaims,- without departing from or exceeding the spirit of theinvention.

The drawing depicts a pair of graphs showing the expansion of aninvestment compound when when added to the simple The addition of any ofthe chlorides of the alkaline earth metals will cause a greater increasein the thermal expansion of the simple investment compositions abovenoted, than the addition 'of a similar amount of any other substanceheretofore known. Also these chlorides investment compositions willproduce castings of greater smoothness and cleaner surfaces. thanheretofore possible. Similar results are noted if these chlorides aredissolved in the water that is added to the usual simple investmentcompositions consisting of dry ingredients since the resulting mixtureis the same. Y

Investments having a thermal expansion of 2% may be fifade by usingabout cristobalite as the filler, the usual binder and about2% of any ofthese chlorides. Equal, or very nearly equal results, in the aboveproperties, may be obtained by the use of any one of the other specifiedchlorides, namely, those of the metals of groups III and II, orammonium, rubidium, and caesium.

The above chlorides are, of course, of diiferent costs, and of slightlydifi'erent physical and chemical characteristics and while their effecton the expansion properties varies b slightly preference would of coursebe given to e least expensive and most refractory one of the group.Preference will at present be given, for these reasons, to the chlorideof barium, followed in order by the chlorides of strontium, magnesium,and calcium. Use will therefore be made of barium chloride as an exampleand state that by the use of this material an investment can be madewhich will meet all of the necessary and desirable requirements set outabove. Besides imparting marked increase in thermal expansion it forms aplastic mass with silica, plaster, and

' water, that, is very smooth and creamy in its form for application tothe wax, thus producing smooth and very desirable reproductions. Thiscomposition hardens to a; cementitious mass which retains its strengthfor days, withstands rapid heating, and high temperatures. It does notadhere to castings and produces smooth, dense castings of suchbrightness that necessitate no pickling in acids, and subsequently verylittle bufiing or grinding. In this last property of producing beautifulcastings it is ideal and the casting produced is often as clean andbright as the original alloy, a property long desired in investmentcompositions. Since this barium chloride salt is by nature similar indensity to the density of silica and plaster it will notsettle thedesired chlorides.

the rare rubidium and caesium also increase the expansion markedly.

While the chlorides of the alkaline earth metals produce the bestresults in meeting the desired requirements of an investmentcomposition, it'is obvious that should these not be available, or shouldthe cost factor change materially due to economic and industrialchanges, or should 'all of the desired requirements not be neededforcertain types of castings, then the chloridesof the other groups will beof marked benefit in aiding the skilled' reproduction of desiredarticles. It may also be possible to secure the desired results by theuse of other, materials or minerals which will react to produce any ofhas been found to increase the thermal expansion of silica-plasterinvestments nearly as much as the other chlorides, and this is no doubtdue to the formation of one of the specified chlorides; namely, calciumchloride. Any other material containing chlorine which can be liberatedto react with the plaster or other added ingredients ments containingother expanding ingredients will generally add its influence in aidingthe increase in thermal expansion as well as adding ,its cleaningefi'ect on'the finished casting.

out ,on storage and will maintain a fine mixture for a long time.

It has also been found that a marked increase in expansion may besecured'by the use of a comparable amount of the chlorides of groups IIIand II of qualitative analysis. results are obtained from the chloridesof group .The best I III metals in the following descending order,

{ill of these are examples of the manner in whidh'the specifiedchlorides may be added to ah investment and should therefore beconsidered in the same light and spirit of the invention.

What is claimed is:

1. An investment composition for casting prepious metals and theiralloys, consisting principally of a. siliceous refractory material and acalcium sulfate binderin such proportions as to give a dental investmentcomposition and containingfrom .1 to 2.0 per cent strontium chloride.

Thus hydrochloric acid

